INA139

f

–3dB

=

1

2 R

p

L

C

L

V

O

f

–3dB

R

L

C

L

3

4

Time

with buffer

without Buffer

INA139, INA169

SBOS181E – DECEMBER 2000 – REVISED DECEMBER 2015

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Typical Applications (continued)

Figure 10 illustrates this concept. A low pass filter can be placed between the OPA340 output and the input to

the ADC. The filter capacitor is required to provide any instantaneous demand for current required by the input

stage of the ADC. The filter resistor is required to isolate the OPA340 output from the filter capacitor to maintain

circuit stability. The values for the filter components will vary according to the operational amplifier used for the

buffer and the particular ADC selected. More information can be found regarding the design of the low pass filter

in the TI Precision Design 16-bit 1-MSPS Data Acquisition Reference Design for Single-Ended Multiplexed

Applications, TIPD173.

Figure 11 shows the expected results when driving an analog-to-digital converter at 1 MSPS with and without

buffering the INA139 or INA169 output. Without the buffer, the high impedance of the INA139 or INA169 will

react with the input capacitance and sample and hold (S/H) capacitance of the analog-to-digital converter and will

not allow the S/H to reach the correct final value before it is reset and the next conversion starts. Adding the

buffer amplifier significantly reduces the output impedance driving the S/H and allows for higher conversion rates

than can be achieved without adding the buffer.

8.2.1.3 Application Curve

Figure 11. Driving an ADC With and Without a Buffer

8.2.2 Output Filter

Figure 12. Output Filter

8.2.2.1 Design Requirements

Filter the output of the INA139 or INA169 devices.

8.2.2.2 Detailed Design Procedure

A low-pass filter can be formed at the output of the INA139 or INA169 devices simply by placing a capacitor of

the desired value in parallel with the load resistor. First determine the value of the load resistor needed to

achieve the desired gain. See the table in Figure 9. Next, determine the capacitor value that will result in the

desired cutoff frequency according to the equation shown in Figure 12. Figure 13 illustrates various combinations

12

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